Methods to remove an EBC from a substrate and to repair a coated component

The present disclosure generally relates to methods for removal of an environmental barrier coating layer(s) from a substrate.

Silicon-based materials are employed for high temperature components of gas turbine engines such as, for instance, airfoils (e.g., blades, vanes), combustor liners, and shrouds. The silicon-based materials may include silicon-based monolithic ceramic materials, intermetallic materials, and composites. For example, silicon-based ceramic matrix composites (CMCs) may include silicon-containing fibers reinforcing a silicon-containing matrix phase.

Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present disclosure.

As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components.

Chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth. As used herein, “RE” refers to a rare earth element or a mixture of rare earth elements. More specifically, the “RE” refers to the rare earth elements of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), or mixtures thereof.

As used herein, “alumina” refers to an aluminum oxide in the form of AlO.

As used herein, “silica” refers to a silicon oxide in the form of SiO. Conversely, “elemental silicon” refers to silicon without any alloying materials present, outside of incidental impurities. It is sometimes referred to in the art as “silicon metal.” Elemental silicon has a melting point of about 1414° C.

As used herein, the term “mullite” generally refers to a mineral containing alumina and silica. That is, mullite is a chemical compound of alumina and silica with an alumina (AlO) and silica (SiO) ratio of about 3 to 2 (e.g., within 10 mole % of 3 to 2 of alumina to silica). However, a ratio of about 2 to 1 has also been reported as mullite (e.g., within 10 mole % of 2 to 1 of alumina to silica).

As used herein, the term “substantially free” is understood to mean completely free of said constituent, or inclusive of trace amounts of same. “Trace amounts” are those quantitative levels of chemical constituent that are barely detectable and provide no benefit to the functional or aesthetic properties of the subject composition. The term “substantially free” also encompasses completely free.

In the present disclosure, when a layer is being described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers, unless expressly stated to the contrary. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer.

As used herein, ceramic-matrix-composite or “CMC” refers to a class of materials that include a reinforcing material (e.g., reinforcing fibers) surrounded by a ceramic matrix phase. Generally, the reinforcing fibers provide structural integrity to the ceramic matrix. Some examples of matrix materials of CMCs can include, but are not limited to, non-oxide silicon-based materials (e.g., silicon carbide, silicon nitride, or mixtures thereof), oxide ceramics (e.g., silicon oxycarbides, silicon oxynitrides, aluminum oxide (AlO), silicon dioxide (SiO), aluminosilicates, or mixtures thereof), or mixtures thereof. Optionally, ceramic particles (e.g., oxides of Si, Al, Zr, Y, and combinations thereof) and inorganic fillers (e.g., pyrophyllite, wollastonite, mica, talc, kyanite, and montmorillonite) may also be included within the CMC matrix.

Some examples of reinforcing fibers of CMCs can include, but are not limited to, non-oxide silicon-based materials (e.g., silicon carbide, silicon nitride, or mixtures thereof), non-oxide carbon-based materials (e.g., carbon), oxide ceramics (e.g., silicon oxycarbides, silicon oxynitrides, aluminum oxide (AlO), silicon dioxide (SiO), aluminosilicates such as mullite, or mixtures thereof), or mixtures thereof.

Generally, particular CMCs may be referred to as their combination of type of fiber/type of matrix. For example, C/SiC for carbon-fiber-reinforced silicon carbide; SiC/SiC for silicon carbide-fiber-reinforced silicon carbide, SiC/SiN for silicon carbide fiber-reinforced silicon nitride; SiC/SiC—SiN for silicon carbide fiber-reinforced silicon carbide/silicon nitride matrix mixture, etc. In other examples, the CMCs may be comprised of a matrix and reinforcing fibers comprising oxide-based materials such as aluminum oxide (AlO), silicon dioxide (SiO), aluminosilicates, and mixtures thereof. Aluminosilicates can include crystalline materials such as mullite (3AlO2SiO), as well as glassy aluminosilicates.

In certain embodiments, the reinforcing fibers may be bundled and/or coated prior to inclusion within the matrix. For example, bundles of the fibers may be formed as a reinforced tape, such as a unidirectional reinforced tape. A plurality of the tapes may be laid up together to form a preform component. The bundles of fibers may be impregnated with a slurry composition prior to forming the preform or after formation of the preform. The preform may then undergo thermal processing, such as a cure or burn-out to yield a high char residue in the preform, and subsequent chemical processing, such as melt-infiltration with silicon, to arrive at a component formed of a CMC material having a desired chemical composition.

Such materials, along with certain monolithic ceramics (i.e., ceramic materials without a reinforcing material), are particularly suitable for higher temperature applications. Additionally, these ceramic materials are lightweight compared to superalloys, yet can still provide strength and durability to the component made therefrom. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (e.g., turbines, and vanes), combustors, shrouds and other like components, that would benefit from the lighter-weight and higher temperature capability these materials can offer.

As used herein, environmental-barrier-coating or “EBCs” refers to a coating system comprising one or more layers of ceramic materials, each of which provides specific or multi-functional protections to the underlying CMC. EBCs generally include a plurality of layers, such as rare earth silicate coatings (e.g., rare earth disilicates such as slurry or APS-deposited yttrium ytterbium disilicate (YbYDS)), alkaline earth aluminosilicates (e.g., comprising barium-strontium-aluminum silicate (BSAS), such as having a range of compositions of BaO, SrO, AlO, SiO, or combinations thereof), hermetic layers (e.g., a rare earth disilicate), outer coatings (e.g., comprising a rare earth monosilicate, such as slurry or APS-deposited yttrium monosilicate (YMS)), or combinations thereof. One or more layers may be doped as desired, and the EBC may also be coated with an abradable coating.

The term “defect” as used herein refers to a portion of the protective layers, substrate, or both exposed to the environment due to damage.

Reference will now be made in detail to present embodiments of the disclosure, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the disclosure.

Although silicon-containing substrates exhibit desirable high temperature characteristics, such substrates can suffer from rapid recession in combustion environments. For example, silicon-containing substrates are susceptible to volatilization upon high-temperature exposure to reactive species such as water vapor. In such cases, coatings are used to protect the silicon-containing substrates. Silicon-containing substrates, such as CMCs, may have multiple protective coating layers on its surface, such as a silicon bondcoat, di-silicate EBCs, mono-silicate EBCs, or a combination thereof. These protective layers help to prevent the degradation of silicon-containing substrates in a corrosive water-containing environment by inhibiting the ingress of water vapor and the subsequent formation of volatile products such as silicon hydroxide (e.g., Si(OH)). Several additional layers, such as an abradable layer, may also be deposited on the EBC to provide specific functionality to CMC components. Thus, the protective layers may enhance the high temperature environmental stability of silicon-containing substrates. Other desired properties for the EBC include a thermal expansion compatibility with the silicon-containing substrate, low permeability for oxidants, low thermal conductivity, and chemical compatibility with the thermally grown silicon-based oxide.

During service, one or more of these protective layers may suffer from damage, such as in the form of a defect. If an EBC experiences a localized spall or a pinhole defect, the underlying substrate may be subject to material loss resulting from water vapor-induced volatilization and subsequent surface recession during operation. If allowed to grow unmitigated, such material loss may reduce the load-bearing capability of the component, disrupt airflow, or even progress to through-thickness holes, which may adversely affect the operating performance and durability of the machine.

In order to repair a worn or damaged environmental barrier coating, a process for removing any existing environmental barrier coating is desired to produce a refreshed surface on the substrate for the formation of a replacement bondcoat, a replacement environmental barrier coating, or both.

Methods are generally provided for removing an environmental barrier coating from a coated component. The method includes contacting the coated component with an etchant liquid, with the etchant liquid comprising hydrogen fluoride and a solvent. The etchant liquid may be further tailored based on the chemistry of layers between the substrate of the coated component and the environmental barrier coating, such as the chemical composition of a bondcoat (and thermally grown oxide layer, if present) therebetween.

Referring to, an exemplary coated componentis shown that include a silicon-containing substratehaving a surfacewith a coating systemthereon. In one particular embodiment, the silicon-containing substrateis formed from a CMC material. Similarly, referring to, an exemplary coated componentis shown that include a silicon-containing substratehaving a surfacewith a coating systemthereon. In one particular embodiment, the silicon-containing substrateis formed from a CMC material.

Generally, the coating systemincludes a bondcoaton the surfaceof the substrate. In the embodiments shown, the bondcoatis directly on the surfacewithout any layer therebetween. An EBCis over the bondcoat. In one embodiment, the bondcoatcomprises silicon (e.g., elemental silicon), a silicon-based material (e.g., a silicide), mullite, or a combination thereof. Generally, the bondcoatis relatively thin, such as having a thickness that is 25 micrometers (μm) to 275 μm, such as 25 μm to 150 μm (e.g., 25 μm to 100).

The EBCmay include a plurality of individual EBC layers, with each of the individual EBC layersbeing formed of materials selected from typical EBC or thermal barrier coating (“TBC”) layer chemistries, including but not limited to rare earth silicates (e.g., mono-silicates and di-silicates), aluminosilicates (e.g., mullite, barium strontium aluminosilicate (BSAS), rare earth aluminosilicates, etc.), hafnia, zirconia, stabilized hafnia, stabilized zirconia, rare earth hafnates, rare earth zirconates, rare earth gallium oxide, etc. For example, one individual EBC layermay be include hafnia (e.g., a hafnia layer), alumina (e.g., an alumina layer), or both. Alternatively or additionally, the EBC may include one or more rare earth silicate layers (e.g., a rare earth disilicate layer, a rare earth monosilicate layer, or both). In one particular embodiment, the EBCmay include a hermetic layer.

As stated above, the method of removal of the EBCmay be tailored depending on the composition of the bondcoat(and thermally grown oxide layer, if present). Suitable methods of removing the EBCare generally provided in the following description, with the methods tailored to the particular chemical composition of the coating system(e.g., the type of bondcoat).

In the embodiment of, by way of non-limiting example, the bondcoatcomprises a silicon-containing material, such as elemental silicon, a silicide, or a combination thereof. For example, the bondcoatmay comprise at least 75% by weight of elemental silicon. This may include at least 95% by weight of elemental silicon. In one embodiment, the bondcoatmay consist essentially of elemental silicon, so as to be substantially free from any other material. In other embodiments, the bondcoatmay include at least 75% by weight of elemental silicon (e.g., at least 95% by weight of elemental silicon), with the balance being another bondcoat material (e.g., a silicide).

When the bondcoatcomprises a silicon-containing material, a thermally grown oxide (“TGO”) layeris present on the surfaceof the bondcoatas shown in. Thus, the bondcoatand the TGO layerare between the silicon-containing substrateand the EBC. The TGO layermay form on the surfaceof the bondcoatwhen silicon in the bondcoatoxidizes upon exposure to air. For example, the TGO layermay be a layer of silicon oxide (sometimes referred to as “silicon oxide scale” or “silica scale”), during exposure to oxygen (e.g., during manufacturing and/or use) of the coated component.

When a TGO layeris present, the coated componentmay be contacted with an etchant liquid, as shown in, that includes 5% by volume to 70% by volume of hydrogen fluoride and a solvent. In particular embodiments, the etchant liquidcomprises 25% by volume to 49% by volume of the hydrogen fluoride so as to be a commercial grade of a hydrogen fluoride solution that is readily available. Generally, the hydrogen fluoride reacts with silicon oxide in the TGO layer. This reaction weakens the chemical bonds in the TGO layer, causing the TGO layerand any overlying layers (i.e., the EBCin the embodiment shown) to be removed from the surfaceof the bondcoat. Conversely, the hydrogen fluoride does not react with the underlying bondcoatnor the substrate, which both are silicon-containing. That is, the bondcoatis not removed from the surfaceof the substrate.

In one particular embodiment, the solvent within the etchant liquidis water. For example, the etchant liquidmay consist essentially of hydrogen fluoride in water so that no other compositions are present therein other than unavoidable trace amounts. Alternatively, the etchant liquidmay further include another acid (e.g., hydrogen chloride, nitric acid, fluorosilicic acid (HSiF), hydrogen peroxide, or mixtures thereof), a wetting agent, etc.

For example, a second acid may be present up to 5% by volume (e.g., 0.1 to 5% by volume) in the etchant liquid. In one embodiment, the second acid may be nitric acid added to the HF, depending on the type of silicon-containing substrateas nitric acid would also etch out and remove any elemental Si in the silicon-containing substrate. For instance, a silicon-containing substrateformed of a chemical vapor infiltrated (CVI) CMC or a polymer infiltration pyrolysis (PIP) CMC is substantially free from any elemental Si. Thus, nitric acid could be used to remove a bondcoatover a silicon-containing substratethat is a CVI CMC. In an alternative embodiment, where elemental Si is present in the silicon-containing substrate(e.g., a melt infiltrated (MI) CMC), the etchant liquidmay be free from nitric acid to avoid etching of the elemental Si in the silicon-containing substrate.

When present, the wetting agent may contain fluorine, such as a perfluoralkane sulfonic acid quaternary ammonium salt, a perfluoralkane carboxylic acid salt, an alkoxylation product of a perfluoralkane sulfonamide, or the like. The wetting agent may be present up to 5% by volume (e.g., 0.1 to 5% by volume).

The coated componentmay be contacted with the etchant liquidhaving a treatment temperature of 20° C. to 60° C. so as to remain in a liquid state during the chemical etch of the TGO layer. As shown in, the coated componentmay be submerged with the etchant liquid.

As shown in, an intermediate componentis formed after removal from the etchant liquid(). By way of a non-limiting example, the etchant liquidmay be washed with a rinsing fluid (e.g., water, alcohols, acetone, or the like), the intermediate componentmay be heated at a drying temperature (e.g., 30° C. to 80° C.) under vacuum or flowing inert gas to volatilize the rinsing fluid, or both washing and heating may be performed. The intermediate componenthas an exposed surfaceof the bondcoaton a silicon-containing substratewith the TGO layerand the EBC() having been removed therefrom. This intermediate componentmay then be transformed into a repaired component by formation of a new, replacement EBC on the exposed surfaceof the bondcoat. In one embodiment, the exposed surfaceof the bondcoatofmay form a replacement TGO layer upon exposure to oxygen (air).

Referring to, a repaired componentis shown having a replacement EBCformed on the intermediate componentof. As shown, the replacement EBCincludes a plurality of individual layers. The replacement EBC, along with the individual layers, may be similar or different to the original EBC() in composition, thickness, layering order, or other variables, as desired. The replacement EBCmay be formed according to any suitable method.

shows a diagram of a methodfor removing an EBC from a coated component, such as shown in. The methodgenerally includes, at, contacting the coated component with an etchant liquid, such as shown in, to remove the EBC and expose a bondcoat. Thus, the stepmay form the intermediate componentof. Optionally, at, a replacement EBC, such as the replacement EBCshown in, may be formed on the exposed bondcoat to form a repaired component, such as the repaired componentshown in.

In another embodiment, the bondcoatmay be removed from the intermediate component() prior to forming replacement layers thereon. Referring to, the bondcoatis shown being removed. More specifically, a mechanical etchantis illustrated as contacting with the intermediate componentto remove the bondcoatto expose the surfaceof the silicon-containing substrate. By way of a non-limiting example, the bondcoatmay be mechanically blasted with a plurality of particlesfor removal of the bondcoat. As shown, the plurality of particlesmay be sprayed via a spray gunthat is moved over the surfaceof the bondcoatto direct the plurality of particlesthereto with sufficient force to remove the bondcoat. In certain embodiments, the plurality of particlesmay be grit particles, glass particles, metal particles, or a mixture thereof. Thus, the surfaceof the silicon-containing substratemay be exposed after removal of the bondcoat.

After removal of the bondcoat, a repaired component can be created by formation of a new, replacement bondcoat and a new, replacement EBC on the exposed surfaceof the silicon-containing substrate. Referring to, a repaired component′ is shown having a replacement bondcoatand a replacement EBC. In the embodiment shown in, the replacement bondcoatis directly on the surfaceof the silicon-containing substrate, and the replacement EBCis directly on the surfaceof the replacement bondcoat.

The replacement bondcoatmay be formed according to any suitable method. The replacement bondcoatmay be similar or different as the original bondcoat() in composition, thickness, or other variables, as desired. For example, the replacement bondcoatmay be formed with a silicon-containing material (e.g., elemental silicon, a silicide, etc.), mullite, or a combination thereof. In one embodiment, the replacement bondcoatmay comprise mullite (e.g., at least 50% by weight mullite, such as at least 75% by weight mullite) to increase the operating temperature from the original bondcoatof. In such an embodiment, the repaired component′ may be substantially free from a TGO layer between the replacement bondcoatand the replacement EBC.

As discussed above with respect to, the replacement EBCin the repaired component′ of, along with the individual layers, may be the same or different as the original EBC() or may be different in composition, thickness, layering order, or other variables, as desired. The replacement EBCmay be formed on a surfaceof the replacement bondcoataccording to any suitable method.

shows a diagram of a methodfor removing an EBC from a coated component, such as shown in. The methodgenerally includes, at, contacting the coated component with an etchant liquid, such as shown in, to remove the EBC and expose a bondcoat. Thus, the stepmay form the intermediate componentof. Optionally, at, the bondcoat may be removed to expose a surface of a silicon-containing substrate, such as shown in. At, a replacement bondcoat, such as the replacement bondcoatshown in, may be optionally formed on the exposed surface of the silicon-containing substrate. At, a replacement EBC, such as the replacement EBCshown in, may be formed on the replacement bondcoat to form a repaired component, such as the repaired component′ shown in.

In the embodiment of, the coating systemincludes a bondcoatthat comprises mullite. For example, the bondcoatmay comprise at least 50% by weight of mullite (e.g., at least 75% by weight of mullite). In one embodiment, the bondcoatmay consist essentially of mullite, so as to be substantially free from any other material. In other embodiments, the bondcoatmay include at least 50% by weight of mullite (e.g., at least 75% by weight of mullite) with the balance being another bondcoat material (e.g., a silicon-containing material, such as described above).

When the bondcoatcomprises a mullite, the coated componentmay be substantially free from any TGO layer on the surfaceof the bondcoat. As such, the EBCmay be directly on the surfaceof the bondcoat, as shown in.

Since the coated componentofis substantially free from any TGO layer, the etchant liquid requires additional components to weaken the chemical bonds between the bondcoatand the EBC. Referring to, the coated componentmay be contacted with an etchant liquidthat includes phosphoric acid and 3% by volume to 70% by volume of hydrogen fluoride in a solvent. In particular embodiments, the etchant liquidcomprises 5% by volume to 49% (e.g., 25% by volume to 49% by volume) of the hydrogen fluoride so as to be a commercial grade of a hydrogen fluoride solution that is readily available. In addition to hydrogen fluoride, the etchant liquidfurther includes phosphoric acid to help weaken the chemical bonds of the bondcoat, EBC, or both. For example, the etchant liquidmay include 10% by volume to 60% by volume of the phosphoric acid in an aqueous solution, such as 30% by volume to 40% by volume of the phosphoric acid in an aqueous solution. One example that might be relevant would be to use an etching solution containing about 37% by volume of the phosphoric acid, which may be combined with about 10% by volume hydrogen fluoride in an aqueous solution to etch mullite. The rate of etch may be controlled by selecting the concentration of the hydrogen fluoride and the phosphoric acid, as lower concentrations within the solvent may slow the etch rate and higher concentrations may increase the etch rate.

Generally, the hydrogen fluoride and phosphoric acid reacts with mullite in the bondcoat, the material in at least one layer of the EBC, or both. This reaction weakens the chemical bonds in the bondcoat, the EBC, or both, leading to either removal of the bondcoator allowing for easier removal in subsequent etching. Conversely, the hydrogen fluoride and phosphoric acid does not react with the underlying silicon-containing substrate.

In one particular embodiment, the solvent within the etchant liquidis water. For example, the etchant liquidmay consist essentially of hydrogen fluoride and phosphoric acid in water so that no other compositions are present therein other than unavoidable trace amounts. Alternatively, the etchant liquidmay further include another acid (e.g., hydrogen chloride, nitric acid, fluorosilicic acid (HSiF), hydrogen peroxide, or mixtures thereof), a wetting agent, etc.

For example, a third acid may be present up to 5% by (e.g., 0.1 to 5% by volume). in the etchant liquid. In one embodiment, the third acid may be nitric acid added to the HF, depending on the type of silicon-containing substrateas nitric acid would also etch out and remove any elemental Si in the silicon-containing substrate. For instance, a silicon-containing substrateformed of a chemical vapor infiltrated (CVI) CMC or a polymer infiltration pyrolysis (PIP) CMC is substantially free from any elemental Si. Thus, nitric acid could be used to remove a bondcoatover a silicon-containing substratethat is a CVI CMC. In an alternative embodiment, where elemental Si is present in the silicon-containing substrate(e.g., a melt infiltrated (MI) CMC), the etchant liquidmay be free from nitric acid to avoid etching of the elemental Si in the silicon-containing substrate.

Additionally or alternatively, when present the wetting agent may contain fluorine, such as a perfluoralkane sulfonic acid quaternary ammonium salt, a perfluoralkane carboxylic acid salt, an alkoxylation product of a perfluoralkane sulfonamide, or the like. The wetting agent may be present up to 5% by volume (e.g., 0.1 to 5% by volume).

The coated componentmay be contacted with the etchant liquidhaving a treatment temperature of 20° C. to 60° C. so as to remain in a liquid state during the chemical etch. As shown in, the coated componentmay be submerged with the etchant liquid.

After removal from the etchant liquid, the resulting intermediate component′ shown in, includes the chemically-weakened bondcoat′ and the chemically-weakened EBC′. Then, as shown in, the chemically-weakened bondcoat′ and the chemically-weakened EBC′ (together defining a chemically-weakened coating′) may be removed from the intermediate component′ () prior to forming replacement layers thereon. Referring to, the chemically-weakened bondcoat′ and the chemically-weakened EBC′ are shown being removed via a mechanical etchantbeing contacted with the intermediate component′ to remove the chemically-weakened bondcoat′ to expose the surfaceof the silicon-containing substrate. For example, the chemically-weakened bondcoat′ may be mechanically blasted with a plurality of particlesfor removal of the chemically-weakened bondcoat′. As shown, the plurality of particlesmay be sprayed via a spray gunthat is moved over the chemically-weakened bondcoat′ and the chemically-weakened EBC′ to direct the plurality of particlesthereto with sufficient force to remove the chemically-weakened bondcoat′ and the chemically-weakened EBC′. In certain embodiments, the plurality of particlesmay be grit particles, glass particles, metal particles, or a mixture thereof. Thus, the surfaceof the silicon-containing substratemay be exposed after removal of the chemically-weakened bondcoat′ and the chemically-weakened EBC′. This intermediate component′ may then be transformed into a repaired component by formation of a new, replacement bondcoat and a new, replacement EBC on the exposed surfaceof the silicon-containing substrate.